Patent Literature 1 has been known as the prior art with regard to spin welding. This aims to reduce a so-called “through transfer phenomenon” that causes a bent or wrinkle on a mounting substrate 6 in the process of fixing a fixture 2 to the mounting substrate 6 by rotational welding. In a rotational welding resin member of Patent literature 1 having a columnar portion and a disk-shaped flange portion integrally provided at one end of the columnar portion, the wall thickness of the flange portion is gradually decreased from the center toward the outer circumference.